Tetramethyl lead process



Nov. 26, 1968 R. H. FEEHS 3,413,323

TETRAMETHYL LEAD PROCESS Filed June 1, 1966 l] I I N {:111 {I I ]I NI 4 no 2 2I In I INVENTOR RICHARD H. FEEHS BY W8. PM

ATTORNEY United States Patent 3,413,328 TETRAMETHYL LEAD PROCESS Richard H. Feehs, Woodstown, N.J., assignor to E. 1. du Pont de Nemours and Company, Wilmington, Del, a corporation of Delaware Filed June 1, 1966, Ser. No. 554,419 3 Claims. (Cl. 260-437) This invention relates to a method of improving the yield of tetramethyl lead and more particularly to a method of recovering quantities of tetramethyl lead heretofore lost in the waste gases.

Tetramethyl lead is manufactured commercially by the reaction of methyl chloride with a sodium-lead alloy. Methyl chloride is fed, usually in excess, to a pressure reactor containing the alloy. When the reaction is complete, the reactor is vented, whereby most of the unreacted methyl chloride is removed. The residual reaction mass in the reactor comprising tetramethyl lead, NaCl, lead and some absorbed methyl chloride is discharged to a steam still where the tetramethyl lead is steam stripped from the solid products, and the distillate is condensed and collected in a receiver. The steam distillation train communicates with the atmosphere through a duct system which is advantageously under a slight vacuum to facilitate flow through it.

It has now been discovered that substantial quantities of tetramethyl lead are lost during venting of the distillate receiver in the production of tetramethyl lead and that the yield can be substantially improved by the process which comprises recovering additional quantities of tetramethyl lead from the gases vented from the distillate receiver by (1) Contacting said gases at a temperature of 20 to 80 C. and a pressure of 0.75 to 1.25 atmospheres with an absorbent oil which (a) dissolves at least one part of tetramethyl lead per 100 parts of oil at said temperature and pressure,

(b) is chemically inert to tetramethyl lead and the carrier gas, and

(c) has a boiling point of at least 200 C.,

(2) Separating the tetramethyl lead-containing oil from the residual gas stream,

(3) Distilling the tetramethyl lead from the oil at a temperature not in excess of 100 C. and

(4) Recovering the tetramethyl lead from the distillate.

Since the cooler used to condense the vapors from the steam distillation is generally operated at about to 35 C., it is quite surprising that a significant amount of tetramethyl lead (B.P. 110 C.) tends to escape from the system along with the non-condensable hydrocarbon gases, air and other volatiles that may be present. In accordance with this invention it has been found that these losses can be substantial, sometimes amounting to as much as about 4% of the methylation yield. The process of this invention provides an efficient process for economically recovering this added tetramethyl lead yield. Prior to this invention it was not realized that such substantial amounts of tetramethyl lead were being lost or that a sufficiently eflicient process could be developed to make it economically feasible to recover this yield.

Tetramethyl lead is commonly prepared and recovered in the presence of a thermal stabilizer such as toluene, which, being comparably volatile, codistills with and tends to be lost equally with tetramethyl lead during venting of the distillate receiver. The steam-tetramethyl lead condensate is often collected in an ethylene dihalide pool in the receiver which further dilutes the tetramethyl lead for safe handling. Some ethylene dihalide, especially the relatively 3,413,328 Patented Nov. 26, 1968 volatile dichloride, tends to be lost with tetramethyl lead and thermal stabilizer in the vent gases. In accordance with this invention, loss of such valuable anti-knock blending agents is also minimized or prevented.

The drawing schematically illustrates a typical embodiment of the invention involving a countercurrent flow two-staged absorption system integrated with steam stripping and recycling systems. This embodiment utilizes an absorption train to remove tetramethyl lead from the vent gas stream with an absorbent oil, and a stripping train to recover both tetramethyl lead and the oil in useable form, for example tetramethyl lead suitable for antiknock blending and oil suitable for reuse in the absorption train. As more fully described below, the tetramethyl lead-containing oil is continuously withdrawn from the absorber, the oil stream is continuously and countercurrently contacted with the steam in a steam still, the resulting tetramethyl lead steam distillate is continuously taken off overhead, and the tetramethyl lead-free oil is continuously withdrawn from the still, separated from residual water and returned to the absorber.

By this method substantially all the tetramethyl lead can be recovered from the vapor streams normally encountered in its manufacture. Other valuable oil-soluble ingredients often contained in the by-product gas streams, such as ethylene dichloride and toluene, can also be recovered along with the tetramethyl lead. The absorbent oil that remains after steam-stripping the tetramethyl lead therefrom is readily separated from the residual steam condensate by decantation or distillation and recycled for reuse in the absorption step. The overall oil loss is loW, normally less than 0.1%. Thus, this process is useful and efiicient for recovering valuable materials that are otherwise lost in waste streams and, by employing readily regeneratable absorbents, is simple, economical and easy to operate.

The overall absorption and recovery process is adaptable to batch or continuous operations, and requires no special equipment. It is preferred, however, to utilize countercurrent scrubbing equipment according to wellknown principles and techniques for effecting gas-liquid contact, including multistage as well as single-stage operations. Simple one-stage extraction can be repeated as many times as may be necessary to achieve the desired degree of extraction. For example, the gaseous tetramethyl leadcontaining mixture can be passed countercurrently through a series of oil scrubbers arranged so that the scrubbing solutions are in increasing order of freshness, the freshest solution being last in the series.

Tetramethyl lead-containing vapor streams which can be treated economically according to this invention contain a minor proportion of tetramethyl lead and a major proportion of a normally paseous carrier such as methyl chloride, hydrocarbons including methane, ethane and butane, hydrogen, nitrogen, air or mixtures thereof. The ration of tetramethyl lead to carrier gas, which very widely in practice, is not critical, but usually the vent gas stream will contain at least about 1% by weight of tetramethyl lead. Compositions containing a major amount of the lead compound are operable but are usually more economically treated by other methods.

Vent gas compositions frequently encountered during steam distillation of MeCl-NaPb reaction masses, produced for example by the process described by Jarvie, Schuler and Sterling in US. Patent No. 3,048,610, typically contain about 45 to 98% by weight air, 1 to 40% tetramethyl lead, and 0.25 to 10% toluene. Such compositions often contain in place of air up to about 40% mehthyl chloride derived from the alkylation reaction, up to about 5% ethylene dichloride from the steam still receiver, and up to about 2% water. They may also contain up to about 3% gaseous hydrocarbons and hydrogen.

The chemical constitution of the absorbent oil is not critical provided it is inert at the operating temperatures to tetramethyl lead, methyl chloride, ethylene dichloride, thermal stabilizers for tetramethyl lead such as toleuene and other Volatile hydrocarbons and steam. The absorbent oil should dissolve at least about 1 part of tetramethyl lead per 100 parts of oil at the operating temperatures and pressures, and preferably at least about parts of tetramethyl lead per 100 parts of the oil. Also, to facilitate subsequent recovery of tetramethyl lead, it should not absorb any substantial amount of the gaseous carrier. Air and the other noncondensable gases normally encountered in tetramethyl lead manufacture are substantially insoluble in most oils at the scrubbing temperatures. Methyl chloride has some solubility in the contemplated oils, but because of its greater volatility, is absorbed to a substantially lesser extent than tetramethyl lead and is readily separated therefrom by distillation.

The absorbent oil should be relatively nonvolatile which facilitates separation of tetramethyl lead therefrom by distillation at temperatures not in excess of about 110 C. Suitable distillation techniques include steam distillation as Well as reduced pressure distillation. Thus, oils whose initial boiling points, as determined by test method D-86 of the American Society for Testing Materials, exceed about 200 C., and preferably exceed about 250 C., in general are suitable. The final boiling point of the oil is not critical but generally does not exceed about 400 C.

The viscosity of the absorbent oil should be low for easy handling, preferably not in excess of about 12 centipoises at 38 C. In general, the less viscous the oil, the more effective is the absorption of the tetramethyl lead from the gas stream.

Various high-boiling absorbent oils for tetramethyl lead are available and suitable for use in this process, including hydrocarbon oils, synthetic esters, glycols and polyether glycols. Hydrocarbon oils are preferred because of their low cost and ready separation from water. Representative hydrocarbon oils are those derived from petroleum by extraction, distillation, or synthetic catalytic methods and include parafiinic, isoparafiinic, naphthenic and aromatic hydrocarbons or mixtures thereof. Examples include the liquid petrolatums or white mineral oils, more specifically exemplified by No. 50 white oil, a refined paraffinic oil having an initial boiling point of about 260 C., a final boiling point of about 370 C. and a viscosity of about 12 cps. at 38 C.; and petroleum process oils known as mineral seal oils having initial boiling points of about 250 to 265 C., final boiling points of about 320 to 360 C., and viscosities of about 1 to 3 cps. at 38 C.

Representative aromatic hydrocarbons may also be used, such as 1,3,5-triethylbenzene (B.P. 218 C.), m-methylnaphthalene (B.P. 240 C.) and a-chloronaphthalene (B.P. 263 C.); but mixtures such as the high aromatic content industrial oils derived from petroleum and boiling in the range of about 210 to 360 C. are preferred because of their low cost.

Representative synthetic oils that may be used are diethylene glycol monobutyl ether acetate (B.P. 247 C.), diethylene glycol dibutyl ether (B.P. 255 C.), triglycol dichloride (B.P. 241 C.), alkyl and aryl monoethers and esters of ethylene glycol and propylene glycol known as Dowanol (boiling betwen about 210 and 350 C.), tetra-(Z-ethylbutyl)orthosilicate (B.P. 238 C./55 mm.), tetra-(2)-ethylhexyl)orthosilicate (B.P. 360 C.) and sulfolane, a cyclic sulfone (B.P. 285 C.).

In general the hydrocarbon oils tend to be immiscible in water, While oxygen-containing oils are miscible with or have appreciable solubility in water. The immiscible oils have the advantage of being more readily separated from the aqueous components resulting from steam distillation and thus are more easily and economically regenerated for reuse in the absorption step. The watermiscible oils on the other hand are generally much less volatile with steam and thus tetramethyl lead can be more cleanly separated therefrom. Solvent phase containing appreciable quantities of water, for example greater than 1% can be readily regenerated water-free, if desired, by distilling off the water; likewise aqueous phases containing substantial quantities of the solvent oil can be distilled to recover the oil.

In general about 1 to 50 parts by weight of absorbent oil is used per part by weight of vent gases being treated. The exact amount employed will depend upon the solvent characteristics of the particular oil with respect to tetramethyl lead.

Referring now to the embodiment illustrated in the drawing, absorber 1 typically is a vertical 9-foot high column having an inside diameter of 42 inches; it is fitted with vent gas inlet line 3, absorbent oil inlet line 4, gas outlet line 5, oil outlet line 6 and is otherwise closed in operation. Absorber 2 typically is a 35-foot high 36-inch diameter column which is adapted to receive gas feed from line 5, it fitted with gas outlet line 7, oil feed line 8 and oil outlet line 9, and is otherwise closed in operation. Absorber 1, as indicated by its height relative to absorber 2, is primarily designed to remove particulate solid and for this purpose is suitably packed to a height of 5 feet with a fouling-free type packing such as Glitch Grid. Absorber 2, the main absorber, is suitably packed with 2-inch steel pall rings to provide a large mass transfer area for gas/ liquid contact.

The stripping train comprises steam stripping column 14 for steaming the tetramethyl lead-oil composition, condenser 17, tetramethyl lead receiver 19, and an oil storage and recovery system comprising separator 23 and oil storage tank 26. Stripper 14 typically is a vertical 36-inch diameter column packed to a 25-foot height with 2-inch steel pall rings; it is fitted with steam inlet line 15, oil inlet line 13, vapor outlet line 16 and oil outlet line 22.

In typical operations with this system, a tetramethyl lead-containing vent gas stream comprising by weight about 45 to 98% air, about 1 to 40% tetramethyl lead, about 0.25 to 10% toluene, about 0 to 5% ethylene dichloride, about 1 to 40% methyl chloride, about 0 to 3% methane, and about 0 to 0.5% hydrogen is fed through line 3 to the bottom of column 1 at a rate of about 250 to 4000 lbs/hr. Simultaneously a water-immiscible absorbent oil as described above, is countercurrently fed to absorber 1 via line 4 at a rate of about 10,000 to 50,000 lbs./hr., and more usually about 35,000 to 45,000 lbs/hr. The temperature within absorber 1 is normally about 20 to C., and preferably about 30 to 70 C. The pressure is generally between about 0.75 and 1.25 atmospheres for convenient operation, and preferably about 1.0 atmosphere.

The oil stream from absorber 1 passes through line 6 to oil storage and recycle tank 10. The scrubbed gas passes via line 5 at a rate of about 250 to 4,000 lbs/hr. to absorber 2 where it countercurrently contacts fresh absorbent oil from line 8 which enters at a rate of about 6,000 to 16,000 lbs./hr., and more usually about 7,000 to 10,000 lbs/hr. As in absorber 1, the temperature and pressure in absorber 2 is about 20 to 80 C. and about-0.75 to 1.25 atmospheres.

The spent gas stream which leaves absorber 2 via line 7 is mainly air and methyl chloride. The oil from absorber 2, which has extracted substantially all the tetramethyl lead, toluene and ethylene dichloride, and a small portion of the methyl chloride from the gas stream, passes "via line 9 to oil tank 10 where it mixes with oil from absorber 1. A portion of the tetramethyl lead-rich oil from tank 10 is recycled via line 4 to absorber 1 at the rate indicated above. The remaining portion of oil in tank 10 passes via line 11, filter 12 and line 13 to the top of steam stripper 14 for countercurrent contact with about 100 C. steam from line 15. The rate of charging oil to stripper 14 is normally about 7,000 to 10,000 lbs./hr., but can conveniently vary from about 6,000 to 16,000 lbs./hr.; the steam flow rate is about 2,000 to 3,000 lbs./hr., but may range from about 1,500 to 3,500 lbs/1m, with the rate being adjusted such that the steam to oil weight ratio is within the range of about 0.1:1 to 1:1, and most preferably about 0.321. As a result of such steaming substantially all of the tetramethyl lead, toluene, ethylene dichloride and methyl chloride vaporizes from the oil, passes overhead through vapor line 16 and water-cooled condenser 17 to receiver 19 where the two-phase distillate stratifies to an upper water layer and a lower tetramethyl lead layer. Receiver 19 may contain blending agents normally used in formulating tetramethyl lead antiknock compositions for shipment and sale such as lead scavenging agents and tetramethyl lead thermal stabilizers, particularly ethylene dibromide and/or ethylene dichloride, in amounts corresponding to about 1-2 moles per mole of tetramethyl lead, and toluene in amounts such that the tetramethyl lead to toluene weight ratio is about 4:1. Alternatively, the lower tetramethyl lead layer of the two-phase steam distillate is drawn oiT via line 21 and blended with the other agents, as described above, normally associated with the finished antiknock compositions in a separate blending tank, not shown. The water layer of the steam distillate is discarged from receiver 19 via line 20.

The hot, spent absorbent oil and the residual water from steam stripper 14 pass via line 22 to oil-water separator 23 where the oil and 'water layers stratify. The water is discharged through line 24, the oil through line 25 to oil storage and recycle tank 26 firom which the recovered oil is recycled to absorber 2 via line 27, filter 28, heat exchanger 30, which cools the oil to about 20 to 80 C., and line 8. If desired, heat exchanger 30 can be eliminated by feeding cool water directly into oilwater separator 23.

It should be understood that the above represents a preferred embodiment and that the apparatus can be varied widely in keeping with the invention. For example, absorber 2 can be an unpacked column and oil from line 8 can be sprayed into the upcoming gas stream for intimate contact. Also absorber 1 can be eliminated or replaced by some other means for screening entrained solids from the gas stream to be treated.

The recovery procedure of this invention is also suitable for recovering tetramethyl lead from gas streams other than the vent gases from the steam distillation of tetramethyl lead. For example, it is equally suitable for recovering tetramethyl lead from the gases obtained by venting the autoclave in the methyl chloride-sodium lead alloy process. These gases typically comprise by Weight 80 to 94% methyl chloride, 1 to 10% tetramethyl lead and to 3% toluene. Such streams may additionally contain up to 2% each of water, air, methane and hydrogen. When ammonia or a volatile amine catalyst such as methylamine is employed for the methyl chloridesodium lead alloy reaction, as described by Pedrotti and Sandy in US. application Ser. No. 293,138, filed July 5, 1963, and now U.S. Patent No. 3,281,442, and

-Pedrotti, Sandy and Tullio in U.S. application Ser. No.

493,927, filed Oct. 27, 1965, the vent gas may also contain up to ammonia or methylamine. Such methyl chloride-based streams can be treated directly in this process or first scrubbed with water, glycol, or a lithium chloride brine, as described by I-I-utton, Murray and Benning in U.S. application Ser. No. 472,364, filed July 15, 1965, to remove the ammonia or methylamine for subsequent recovery. The residual methyl chloride stream, after being scrubbed with oil in accordance with this invention to recover its tetramethyl lead content, can be recycled for use. If desired and necessary, the recovered methyl chloride can first be dried by passing it through solid desiccant or concentrated H 80 before recycling it to the methylation reactor.

The following example, illustrating the novel process disclosed herein, is given without any intention that the invention be limited thereto. All parts and percentages are by weight.

Example A tetramethyl lead-containing vent gas stream resulting from the steam distillation of product from a series of methylations conducted substantially as described by Jarvie, Schuler and Sterling, in U.S. Patent No. 3,048,610, was collected in a common duct. During a three-hour period the gas stream contained on the average about 8% tetramethyl lead, 2% toluene, 18% methyl chloride, 0.5 ethylene dichloride, 0.5% hydrogen and 68% air. Its flow rate through the duct averaged about 1300 lbs/hr. The composition varied with time, depending on the number of reaction masses being steam distilled, on variations in steam still temperatures and on venting operations during this period. Actually, the composition of the gas stream varied as much as 1 to 40% tetramethy lead, 0.25 to 10% toluene, 0 to 5% ethylene dichloride, 1 to 40% methyl chloride, and 45 to 98% air, while the flow rate ranged from 250 to 4000 lbs./hr. Most of the time, however, the tetramethyl lead content ranged from 4 to 10%, the toluene from 1 to 4%, the methyl chloride from 10 to 30%, the ethylene dichloride from 0.2 to 2% and the air plus other inerts from 45 to while the flow rate ranged from 1000 to 1700 lbs/hr.

The tetramethyl lead-containing gas stream defined above was continuously fed to the recovery system illustrated in the drawing. A mineral seal type hydrocarbon oil having an initial atmospheric boiling point of about 260 C. and a viscosity of about 3 cps. at 38 C. was countercurrently fed to the first absorber at a rate of about 41,00 lbs./hr. and to the second absorber at 7,400 to 10,000 lbs./hr., averaging 9,000 lbs/hr. The temperature in the absorbers was about 40 to 60 C. and the pressure was about 1 atmosphere.

The composite oil stream from the absorption zone contained from 0.1 to 0.5% methyl chloride, averaging 0.33%; 0.4 to 3% tetramethyl lead, averaging 1.1%; 0.1 to 0.8% toluene, averaging 0.27%; and 0.02 to 0.1% ethylene dichloride, averaging 0.06%, depending on the duct gas composition and flow rate, and the oil flow rate to the second absorber. The spent gas stream leaving the absorption zone contained an average of 17.8% methyl chloride and 72.8% air. Thus, during the three-hour period the oil had absorbed all the tetramethyl lead, toluene and ethylene dichloride and only about 12.5% of the methyl chloride.

The tetramethyl lead-containing absorbent oil was continuously contacted with 2400 lbs/hr. of 100 C. steam corresponding to 0.27 lb. of steam/lb. of oil. The steam distillate organic layer collected over the three-hour period contained 100 parts of tetramethyl lead, 25 parts of toluene and 5 parts of ethylene dichloride, the methyl chloride having vaporized away.

The oil, which was recovered from the steam still free of tetramethyl lead, toluene and ethylene dichloride, was separated from the aqueous condensate and returned to the absorption zone at the indicated rate.

Although the invention has been described and exemplified by way of specific embodiments, it is to be understood that it is not limited thereto. As will be apparent to those skilled in the art, numerous modifications and variations of the embodiments illustrated above may be made without departing from the spirit of the invention or the scope of the following claims.

The embodiments of the invention in which an eX- clusive property or privilege is claimed are defined as follows:

1. In the process for the production of tetramethyl lead in which methyl chloride and sodium-lead alloy are reacted in a pressure reactor, the reactor is vented thereby removing unreacted methyl chloride, the residual reaction mass is steam distilled, and the distillate containing tetramethyl lead is condensed and collected in a receiver which is vented, the method of improving the yield of tetramethyl lead which comprises recovering additional quantities of tetrarnethyl lead from the gases vented from the distillate receiver by (1) contacting said gases at a temperature of 20 to 80 C. and a pressure of 0.75 to 1.25 atmospheres with an absorbent oil which (a) dissolves at least one part of tetramethyl lead per 100 parts of oil at said temperature and pressure, (b) is chemically inert to tetramethyl lead and any carrier gas, and (c) has a boiling point of at least 2000 C., (2) separating the tetrarnethyl lead-containing absorbent oil from the residual gas stream, (3) distilling tetramethyl lead from the oil at a temperature not in excess of 110 C., and (4) recovering tetramethyl lead from the distillate. 2. The improved process of claim 1 in which the gases vented from the distillate receiver contain at least 1% tetramethyl lead.

3. The improved process of claim 2 in which 1 to 50 parts by Weight of a hydrocarbon oil having a viscosity not in excess of 12 centipoises is used per part of vented gas.

References Cited TOBIAS E. LEVOW, Primary Examiner.

20 H. M. S. SNEED, Assistant Examiner. 

1. IN THE PROCESS FOR THE PRODUCTION OF TETRAMETHYL LEAD IN WHICH METHYL CHLORIDE AND SODIUM-LEAD ALLOY ARE REACTED IN A PRESSURE REACTOR, THE REACTOR IS VENTED THEREBY REMOVING UNREACTED METHYL CHLORIDE, THE RESIDUAL REACTION MASS IS STREAM DISTILLED, AND THE DISTILLATE CONTAINING TETRAMETHYL LEAD IS CONDENSED AND COLLECTED IN A RECEIVER WHICH IS VENTED, THE METHOD OF IMPROVING THE YIELD OF TETRAMETHYL LEAD WHICH COMPRISES RECOVERING ADDITIONAL QUANTITIES OF TETRAMETHYL LEAD FROM THE GASES VENTED FROM THE DISTILLATE RECEIVER BY (1) CONTACTING SAID GASES AT A TEMPERATURE OF 20* TO 80* C. AND A PRESSURE OF 0.75 TO 1.25 ATMOSPHERES WITH AN ABSORBENT OIL WHICH (A) DISSOLVES AT LEAST ONE PART OF TETRAMETHYL LEAD PER 100 PARTS OF OIL AT SAID TEMPERATURE AND PRESSURE, (B) IS CHEMICALLY INERT TO TETRAMETHYL LEAD AND ANY CARRIER GAS, AND (C) HAS A BOILING POINT OF AT LEAST 2000*C., (2) SEPARATING THE TETRAMETHYL LEAD-CONTAINING ABSORBENT OIL FROM THE RESIDUAL GAS STREAM, (3) DISTILLING TETRAMETHYL LEAD FROM THE OIL AT A TEMPERATURE NOT IN EXCESS OF 110*C., AND (4) RECOVERING TETRAMETHYL LEAD FROM THE DISTILLATE. 